(many edits)
[face=SPIonic]oi(/[/face] is masc. pl. nom. of [face=SPIonic]o(/j[/face].
In Elements, Euclid uses '[face=SPIonic]o(/per[/face]' as "the very thing that" as in "[face=SPIonic]o(/per e)/dei dei=cai[/face](the very thing that needed to be shown)".
But this kind of particles change their meaning abruptly according to the gender, number, and cases. In this case the note in Goodwin says its a causal relative pronoun.
Or you could refer to the White's Illustrated Dictionary of Anabasis, page 161, to find [face=SPIonic]o(/sper[/face] 'who certainly(rel. pron.)'.